The present invention relates to a satellite amplifier system adapted to distribute a plurality of received transmission channels flexibly to a plurality of output beams.
In the general case of space missions, evolution of satellite transmissions to users equipped with small send/receive terminals of low capacity implies increasing the receive quality of the onboard segment and increasing the power of the signals retransmitted to the ground. These performance improvements are obtained by increasing the gain of the onboard antennas, which can be achieved only by reducing the dimensions of their coverages on the ground. To cover a particular geographical coverage region on the ground, these coverage reductions necessitate the generation of a plurality of beams or spots in order to sample the geographical region. This kind of multibeam or multispot coverage enables connections with small ground terminals but give rise to the problem of managing onboard capacities and more particularly of allocating received channels to the transmitted beams as a function of:                different traffic densities, and        changing traffic densities.        
Accordingly, in a manner that is known in the art and represented diagrammatically in the architecture 1 shown in FIG. 1, a satellite receives 64 signals each corresponding to one transmission channel and supplies 32 beams. The 64 channels are processed by an input section 2 which:                handles low-noise reception, appropriate frequency conversion and appropriate filtering for each of the 64 transmission channels, and        outputs each of the 64 channels to an amplifier 3.        
A transmission channel corresponds to a transmission frequency band and may correspond to a single carrier or to a set of carriers or sub-channels.
Each transmission channel is amplified by the associated amplifier 3. The amplifiers 3 are high-power amplifiers and generally take the form of traveling wave tube or solid state amplifiers. To provide a plurality of channels for each beam, it is necessary to combine the channels by means of output multiplexers (OMUX) 4. The output multiplexer 4 provided at the output of each amplifier is described in “Satellite Communications Systems” (G. Maral and M. Bousquet, WILEY, second edition, page 411 et seq.). The multiplexer 4 comprises filters and a common guide that is adapted to combine the transmission channels after amplification. In FIG. 1, each output multiplexer 4 receives two transmission channels and supplies one beam signal. Each beam signal is then sent to a source, not shown, such as a horn that radiates toward a reflector, not shown, to form the beam. Thus an architecture of this kind provides two transmission channels per downlink beam.
However, operators do not always have a very clear view of the future distribution of traffic (and therefore of power) over the coverages addressed and therefore need some flexibility for adapting during the service life of the satellite to traffic requirements resulting from the demand for and the success of services in different geographical regions. It is therefore important to be able to route transmission channels to beams flexibly, i.e. so that the total number of channels processed by the payload can be distributed to the various beams in accordance with traffic demand throughout the service life of the satellite. In this sense, the architecture as shown in FIG. 1 provides no flexibility in terms of the number of channels allocated to each beam and requires a number of amplifiers that is imposed by the number of channels to be amplified.
One prior art solution to this problem consists, as shown in FIG. 2, in using a multiport amplifier (MPA) 10 to provide flexible amplification and allocation of 64 transmission channels to 32 beams. The system 10 comprises:                an input section 11 with 64 inputs and 64 outputs, which, like the section 2, handles low-noise reception, appropriate frequency conversion and appropriate filtering for each of the 64 transmission channels,        a switch 12 with 64 inputs and 32 outputs,        a Butler matrix 13 with 32 inputs and 32 outputs,        an inverse Butler matrix 14 with 32 inputs and 32 outputs, and        32 high-power amplifiers 15, each output of the matrix 13 being connected to the corresponding input of the matrix 14 via a high-power amplifier 15.        
The transfer function of the inverse Butler matrix 14 is the inverse of that of the Butler matrix 13.
Thus the input section 11 receives 64 uplink transmission channels and:                handles low-noise reception, appropriate frequency conversion and appropriate filtering of each of the 64 transmission channels, and        outputs the 64 transmission channels to the 64 inputs of the switch 12.        
The switch 12 is a low level (i.e. low power) switch, generally an electromechanical or electronic switch, and merely routes the channels present at its 64 inputs to its 32 outputs and multiplexes them (summation of a plurality of channels at the same output). The complexity of the switch 12 depends on the required flexibility, as reflected in the number of outputs to which each channel may be routed and the number of channels that may be routed to the some output.
The output signals of the switch 12, which may correspond to a plurality of channels, are then sent to the 32 inputs of the Butler matrix 13.
In the Butler matrix 13, which is made up of 3 dB couplers, the signal at each output is a combination of the signals at all the inputs, but the signals coming from the diverse inputs have a predetermined phase, different from one input to another, which means that the input signals may be reconstituted in their entirety after amplification by the amplifiers 15 and passing through the inverse Butler matrix 14. In other words, the 32 output signals of the switch are obtained identically, after amplification, at the output ports of the inverse Butler matrix 14 (identity product of the Butler matrix 13 by the inverse Butler matrix 14). Each amplifier 15 contributes to the amplification of all the signals present at the inputs of the Butler matrix 13. Rated to pass a maximum number of transmission channels, a system 10 of the above kind authorizes any distribution thereof.
Each beam signal obtained at the outputs of the inverse Butler matrix 14 is then sent to a source, not shown, such as a horn that radiates toward a reflector, not shown, to form the beam.
Thus the system 10 provides total flexibility in that it is functionally possible to allocate each of the 64 transmission channels to each of the 32 beams. It is thus functionally possible to place all the amplified transmission channels on a single beam or to distribute them equally between the beams, which are allocated on the upstream side of the Butler matrices by the switch 12.
However, using this kind of solution gives rise to certain problems, in particular the problem of the feasibility of a high order MPA system, which is linked to the feasibility of the Butler matrices and the parallel connection of a large number of power amplifiers, together with a loss of efficiency in terms of the energy efficiency of the system 10 from FIG. 2 compared to the system 1 from FIG. 1.
Thus it is common to encounter up to 64 or more beams to provide the coverage of a continent (multimedia application in the Ka band). In this case, MPA systems with 64 inputs and 64 outputs, or even 64 inputs and 32 outputs, as shown in FIG. 2, are extremely difficult to implement, in particular because of the complexity of the Butler matrices used.
One option is to divide the Butler matrices 13 and 14 by using a plurality of subgroups of smaller Butler matrices. For example, FIG. 3 shows a system of this kind identical to the system 10 from FIG. 2 except that the FIG. 2 matrices 13 and 14 have been divided into two identical sub-matrices with 16 inputs and 16 outputs. This kind of solution is less flexible, however, in that, to avoid overspecifying the power amplifiers, it is not possible to amplify the 64 transmission channels in the same submatrix. To use the same amplifiers as the system 10 from FIG. 2, the number of channels processed by each submatrix must not exceed 32 and the system 12 must therefore divide the 64 channels between two blocks each of 32 channels. This constraint limits the flexibility of the FIG. 3 system compared to that of the FIG. 2 system 10.
Furthermore, it should be noted that, in an MPA type solution, the number of inputs and outputs of the Butler and inverse Butler matrices must be at least equal to the number of beams, to enable connection of all the beam ports to the output of the MPA system. Thus the number of amplifiers required is imposed by the number of beams, which is reflected in high cost and a complex arrangement.
Finally, if all the channels are transmitted via an MPA system, each power amplifier contributes to the amplification of a large number of channels (64 channels per amplifier for the FIG. 2 system 10 compared to the one channel per amplifier for the FIG. 1 system 1). This multichannel operation is reflected in the obligatory operation of the power amplifier far from its saturation point, which is in turn reflected in a loss in its energy efficiency (power delivered/power consumed) and therefore in a significant increase in the power consumed by the system 10 for exactly the same power delivered per transmission channel as for the systems 1 and 10. The increased flexibility of the system 10 compared to the system 1 is therefore reflected in a degraded energy balance.